Magnetoresistive sensors are based on the principle that the resistance of a ferromagnetic material changes when subjected to a magnetic flux. Magnetoresistive sensors have numerous applications including ascertaining shaft rotation parameters (position, acceleration, etc.) in the context of encoders, tachometers, etc. In this regard, U.S. Pat. No. 5,293,125 entitled "Self Aligning Tachometer With Interchangeable Elements For Different Resolution Outputs" assigned to the present assignee is incorporated herein by reference. In these applications, magnetoresistive sensors detect changes, in magnetic fields to measure motion.
FIG. 1 shows one application of the present embodiment in which a magnetic drum 100 includes a peripheral surface 112 having two distinct tracks: an incremental or INC track 116 and an index or Z track 118. The rotary drum 100 is mounted to a shaft 114 which may be connected for example to a motor (not shown). The INC track 116 provides pulses indicating incremental shaft rotation and comprises an alternating series of magnetic north and south poles magnetically imprinted on the drum periphery 112 at a predetermined pitch .lambda. which may be on the order of hundreds of microns, (e.g., 747 microns in the preferred embodiment). Depending on the diameter of the drum 100, the peripheral surface 112 may encode a large number of pulses per revolution, (e.g., 480, 512, 600, etc.) The Z track 118 is used to generate one output pulse per revolution of the drum and thus provides information concerning the number of shaft revolutions and the absolute shaft position. Accordingly, whenever a Z pulse is generated, the motor shaft is known to be at a particular absolute position relative to the magnetoresistive sensor module 120.
Magnetoresistive sensor module 120 and 120' include a plurality of magnetoresistive elements positioned adjacent to and separated by a predetermined gap from the drum peripheral surface 112 as will be described in more detail below. The magnetoresistive INC track 116 has corresponding sensor module 120, and the Z track 118 has corresponding sensor module 120'. Both sensor modules 120 and 120' are connected to signal sensing and conditioning circuitry 122.
Each of the magnetoresistive sensors 120, 120' consists of a glass substrate covered with a thin film permaloy, e.g., a Ni--Fe film, which is photoetched into a pattern of individual elements which are connected to the sensing and signal conditioning circuitry 122 via one or more flexible leads. Reference is made to FIG. 2(a) which is a perspective view of a portion of the magnetized INC track 116 showing the adjacent north and south poles (N, S) on its incremental track and plural magnetoresistive elements 124 including elements MR.sub.1 and MR.sub.2 with connecting nodes A, B, and C, a DC voltage being connected to nodes A and B. As can be seen from the drawing, the magnetoresistive elements are formed parallel to each other and to the north and south poles formed on the peripheral surface of the drum 112. The magnetoresistive elements are typically spaced some fraction of the pitch distance .lambda. separating each adjacent magnetic pole, e.g. .lambda./2 in FIG. 2(a).
FIG. 2(b) shows a current i generated in a linear magnetoresistive strip in response to an orthogonal magnetic field H. As shown in FIG. 2(c), the magnetoresistive strip experiences a drop in electrical resistance R (corresponding to an increase in current i) in the presence of the saturated magnetic field H. More specifically, the electrical resistance R of the thin film magnetoresistive pattern inversely varies in accordance with the strength of magnetic field H which intersects a perpendicular current i running through the magnetoresistive pattern as shown. In theory, the change in resistance R is independent of the polarity of the magnetic field H. However, as will be described in more detail below, this assumption is not reliable in practical magnetoresistive sensor applications.
Referring to FIG. 2(d), the magnetoresistive sensor elements MR.sub.1 and MR.sub.2 are conventionally connected in a resistive bridge array so as to provide differential outputs e.g., the output signal is taken from node C. Note as the drum 100 rotates the magnetic pole pattern on INC track 116 past the magnetoresistive sensor elements MR.sub.1 and MR.sub.2, an AC output generated at bridge circuit node C corresponds to the movement of the magnetic pole pattern and therefore the rotation of the drum 100.
Magnetoresistive sensors are designed to increase the output voltage level and to improve the temperature properties of the device by making bridge connections between several elements. Two phase outputs (i.e., A and B phases) are typically obtained from the sensor by offsetting the magnetoresistive sensor's pattern of elements from the north-south pole pattern on the INC track 116 of the magnetic drum 100 by one quarter of the pole pitch .lambda.. FIG. 3 illustrates a simple configuration of magnetic resistive elements a.sub.1, b.sub.1, a.sub.2, and b.sub.2 positioned parallel to and above the magnetic pole surface corresponding magnetic field lines between four adjacent poles. One phase or channel of a magnetoresistive sensor comprises two magnetoresistive strips displaced an odd multiple of a half pole pitch from each other which in the layout in FIG. 3 is 3.lambda./2. As the drum 100 rotates one pole pitch .lambda., the one channel sensor output (which can be assumed for simplicity to be an approximately sinusoidal output waveform) completes one cycle having a particular phase A. A quadrature signal (phase B) which is 90.degree. out of phase from phase A is generated by the B channel magnetoresistive sensor elements B1 and B2 which are formed on the same substrate as elements A1 and A2 but displaced an odd multiple of a quarter pole pitch from the first pair A1, A2.
The phase A and phase B bridge outputs are typically amplified and converted into square waveforms using conventional comparators or other zero crossing detection methods. The square waveforms for phases A (.THETA..sub.A) and B (.THETA..sub.B) shown in FIG. 4 are in a quadrature relationship, i.e. .THETA..sub.A leads .THETA..sub.B by 90.degree.. By combining the two quadrature phases .THETA..sub.A and .THETA..sub.B in an exclusive-OR gate, a single channel output of twice the frequency of the quadrature signals is obtained. This means that the output resolution of the magnetoresistive sensor is "doubled" without any increase in the number of magnetized poles formed on the rotary drum peripheral surface. Such sensors are referred to as frequency doubling sensors and achieve higher resolution without having to increase the manufacturing accuracy that would otherwise be required to make smaller magnetized poles on the rotary drum. In theory, additional exclusive-OR outputs may be recombined using further exclusive OR-gates to produce even higher resolutions by frequency tripling, quadrupling, etc. Although the present invention may be applied to frequency tripling, quadrupling, etc. embodiments, the present invention is described in the context of a frequency doubling sensor for the sake of simplicity.
A frequency doubling magnetoresistive sensor is shown in FIG. 5. The magnetoresistive sensor includes two sets of five magnetoresistive elements, the first set including elements 5-9 and second set including elements 10-14. The first and second sets of magnetoresistive elements are separated by one drum magnetic pole pitch .lambda.. Each magnetoresistive element within a group is spaced by some fraction of the pitch, e.g., by 3.lambda./8. Magnetoresistive elements 5-9 are connected to power supply Vcc which may be for example 5 volts. The other terminals of magnetoresistive elements 5-9 are connected to output terminals A-E and to terminals of corresponding magnetoresistive elements 10-14 from the second set. The other terminal of magnetoresistive elements 10-14 is connected to ground.
The bridge circuit schematic formed by the magnetoresistive sensor shown in FIG. 5 is illustrated in FIG. 6. Note that the magnetic field H generated by the magnetic poles formed on the drum shown in FIG. 5 is approximated as a sinusoid and assumes that the magnitude of all positive magnetic fields are identical to each other and all negative fields are identical to each other. If it could be further assumed that the positive field has the same magnitude as the negative field and that therefore the negative and positive fields are symmetric, the bridge outputs A-E could be combined to generate a frequency doubled square wave output having a constant period T. In other words, if it could be assumed that the magnetic fields generated from the magnetic pole drum pattern are uniform in magnitude and from pole to pole, the square wave output would be completely uniform.
In practice, the magnetic pole pattern formed on the drum surface is not symmetric, and the positive and negative magnetic fields may have different magnitudes. As a result of the fields generated from the magnetic pattern not repeating exactly from pole to pole, i.e., asymmetries in the detected magnetic field, the square waves generated by the bridge circuit do not have a uniform and constant period. This nonuniformity or variation in the period of the generated square waves is defined as "period jitter" or simply "jitter." More formally, jitter is defined as follows: ##EQU1##
In this definition, jitter is expressed as a percentage of the average period with 0% corresponding to no jitter.
Current encoders which employ frequency doubling magnetoresistive sensors produce jitter as high as twenty-five percent. However, jitter of less than five percent is desirable. Factors that contribute to the magnetic pole asymmetry that causes jitter include:
Hysteresis or bias of the magnetoresistive sensor material which occurs when the sensor material reaches maximum resistance at some magnetic field intensity other than zero. Sensor bias also occurs when the sensor is more sensitive to a magnetic field having one polarity than to a field of the same magnitude in the opposite pole direction. PA1 Variations in the shape of the sensor output signal. Variations in the gap separation between the magnetized drum and the magnetoresistive sensor and third harmonic distortions and imperfections in the magnetized surface cause distortions in the output signal. These are often a function of or related to the direction of the magnetic field. PA1 Misalignment of the sensor with respect to the magnetized pattern. This causes a distortion in the sensor output that manifests itself in part as a variation in the sensed field.
The present invention substantially reduces jitter in magnetoresistive sensors. A magnetoresistive sensor in accordance with the present invention includes one or more magnetoresistive elements positioned in array of magnetoresistive elements for detecting a changing magnetic field having an alternating polarity caused by relative movement between the magnetoresistive elements and a magnetized pattern wherein the one or more complementary magnetoresistive elements are positioned substantially 180.degree. out of magnetic phase thereby compensating for and substantially cancelling the effects of jitter. By placing one or more pairs of complementary elements substantially 180.degree. out of phase, asymmetric magnetic fields, individual physical differences between the sensors, sensor bias/hysteresis, distortion, and other nonuniformities are compensated. Each pair of complementary elements is combined in one leg of a bridge circuit where the output of each leg of the bridge circuit produces a waveform having a substantially constant period.
The present invention encompasses a rotary encoder having a drum having a circumference covered with a magnetic track having a predetermined pitch between magnetic poles and a magnetoresistive sensor. The sensor includes a plurality of magnetoresistive elements positioned opposite the drum and connected in a bridge circuit such that when the drum rotates, an output signal from the bridge indicates a rotary angle of the drum. The magnetoresistive elements include one or more elements positioned at a distance of one pitch from one or more other corresponding elements.
A method of making a magnetoresistive sensor having a plurality of parallel magnetoresistive elements includes positioning first magnetoresistive elements in a first series of locations and positioning one or more complementary magnetoresistive elements in one or more second locations so that the one or more complementary magnetoresistive elements are substantially 180.degree. out of phase of a corresponding one or more of the first magnetoresistive elements. By making the magnetoresistive sensor in this way, signals generated by each corresponding pair of first element and a complementary element may be averaged to cancel nonsymmetric responses thereby reducing jitter.